The lens is of the type that is pushed up the cornea by the lower eyelid upon downgazing for near vision, and settles back, due to gravity, upon looking up for distance vision. This translational movement of the lens relative to the pupil is necessary in order for the pupil to be located behind the upper part of the lens for distance vision, and behind the lower part of the lens for near vision.
A major problem encountered with these lenses is that of stability of their angular orientation, so that after intentional translation, or translation caused by involuntary blinking, the lens will automatically return to its proper orientation, in which the distance-vision half of the lens is indeed the upper half and the near-vision half, the lower half, with the border line between the two halves in a substantially horizontal orientation.
In prior art lenses, attempts were made to solve this problem by making the lower half of the lens much heavier, as it were ballasting it, and also truncating the lower edge of the lens. While the general orientation of the moving lens was helped by the ballasted lower part, final alignment was left to the flat edge portion produced by the truncation, which portion, when coming in contact with the lower eyelid, was supposed to bring about precise orientation.
These lenses, mainly due to their excessive weight and bulk, proved to be very uncomfortable and a significant proportion of patients could not use them at all, or when wearing them, did not derive much benefit from them.
U.S. Pat. No. 4,268,133 (Fischer, et al.) discloses a contact lens which is claimed to be self-orienting without the help of ballasting or truncation as discussed above. This is said to have been achieved by the provision, at a marginal region intended to be the topmost region when the lens is properly oriented, of one or more raised or recessed characters (such as, e.g., x'es), care being taken for these characters not to intersect the edge of the lens.